Electronic devices, such as portable electronic devices, frequently comprise a user interface that allows a user to enter control commands or other data. Many conventional user interfaces which have been used for several decades involve hardware keys which include mechanically moveable elements. With a view to device aesthetics, there have been attempts to change the construction of electronic devices, and in particular of portable electronic devices, so as to reduce the number of mechanically moveable elements which are provided on the electronic device. One route which has been pursued has been to utilize touch sensitive screens. Such touch sensitive screens may traditionally be based on resistive or capacitive sensing techniques. In order to address some of the drawbacks associated with touch-sensitive displays, there have been proposals to use proximity sensors which do not require a user to physically touch the display. One example for such a proximity sensor is described in WO 2010/069410 A1.
Even portable electronic devices which now widely use a touch-sensitive or proximity sensing display as one input device frequently have one or several input keys separate from the display, for purposes such as switching on or off the device, menu control functions, loudspeaker volume control functions, scrolling functions, or similar. Today, such keys may still frequently be implemented using mechanically moveable elements. Besides the obvious aesthetic drawbacks of having such keys in an otherwise seamless design, mechanical input elements may be prone to wearing out over time and/or the mechanical input elements may be subject to the risk of being ruined by dirt or moisture entering the rather large openings in the device housing. Such large openings are necessary to accommodate the traditional elements of mechanical keys.
While it is conceivable to utilize input keys which can be seamlessly integrated into device housing by sensing depression of a part of the device housing, for example using capacitance sensing, there are some limitations associated with such a design. For illustration, there may be limitations with regard to positioning a key which requires a portion of the device housing to be depressed. More specifically, it may be difficult to position such a key close to a corner or edge of the device housing, because the corner or edge may provide the device surface with enhanced stiffness. Reliable detection of key actuation may then become a considerable challenge.